Null exception of private method of spring controller in AOP scenario

Yang Xiaogui 2020-11-09 19:22:33
null exception private method spring

First say the conclusion and then analyze the reasons :

If Controller There is AOP Dynamic agent behavior such as aspect , also Controller Yes service bean, So if the method is private modification , This method is calling service A null pointer exception is reported , Change it to public/protected/ In package permissions That's all right. .

Cause analysis :

spring How to realize dynamic agent

spring have access to jdk proxy and cglib Two ways to realize dynamic proxy , among cglib Faceted logic weaving can be implemented at compile or run time

  • jdk proxy It is applicable to the proxy class, which is the implementation of an interface class
  • cglib Applicable to the proxy class, which does not implement an interface , Methods of a delegate class can only be overridden by inheritance , Realize logic weaving , Generally, there are EnhancerBySpringCglib word

java Inheritance rules ,private Cannot inherit does not mean that it cannot be called

Define a Student class :
public class Student {
public void eat(String name) {
System.out.println(name+" I am eating ...");
private void sleep(String name){
System.out.println(name+" Sleeping on the sly ...");
Write a method interceptor , Simulated section :
public class TargetInterceptor implements MethodInterceptor {
public Object intercept(Object obj, Method method,
Object[] params, MethodProxy proxy) throws Throwable {
System.out.println(" Before calling ");
Object result = proxy.invokeSuper(obj, params);
System.out.println(" After calling ");
return result;
stay Student Add one to the class main Method :
public static void main(String[] args) throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
// Create bytecode enhancer 
Enhancer enhancer =new Enhancer();
// Set parent class 
// Set the callback function 
enhancer.setCallback(new TargetInterceptor());
// Create a proxy class , It's equivalent to EnhancerBySpringCglib class 
Student student=(Student)enhancer.create();
Method eat = student.getClass().getMethod("eat", String.class);
eat.invoke(student," Wang Er Gan Zi ");
// Notice here is access Student Class method , The proxy class does not inherit to this method 
Method sleep = Student.class.getDeclaredMethod("sleep", String.class);
sleep.invoke(student," Wang Er Gan Zi ");
student.sleep(" Wang Er Gan Zi ");
Output :
 Before calling
Wang Er Ganzi is eating ...
After calling
Wang Er Gan Zi is sleeping secretly ...
Wang Er Gan Zi is sleeping secretly ...
explain :
 call eat Method , The proxy class was called , The proxy class executes the interceptor first ( section ) Logic , And then call the real class. , Because the proxy class inherits and rewrites Student Of eat Method .
Why? sleep No way , because sleep The method is private , Cannot be inherited or overridden , When called through reflection , The subclass did not look for the parent class ,
Call it directly if you can find it , Because the parent reference points to a child object , And our main The way is to write it in Student The inside of the class ,
Can be reflected , It can also be called directly , If main Write on the outside , You can call it through reflection .

spring Initialization process

I believe you know why , call controller Method , Call first proxy class , But now proxy classes can't inherit and override private Method , That causes reflection to call the parent class of the proxy class directly private Method , This private Method is called service bean, That is, the properties of the proxy class , by null, Journal of nature NPE, So here comes the question , Why? spring Do not inject proxy classes service bean, Or is it necessary to inject ? Let's analyze spring The initialization process , Why didn't you do it

本文为[Yang Xiaogui]所创,转载请带上原文链接,感谢

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